The present invention relates to a method and apparatus for forming hollow blow-molded containers of a biaxially oriented thermoplastic material, and more particularly to methods and apparatus for forming thin-walled plastic containers configured to accommodate partial evacuation without adverse effects on their appearance or strength.
Lightweight, thin-walled containers made of thermoplastic materials such as polyester resin and thermoplastic polymers containing at least 50% by weight polymerized nitrile-group-containing monomer (hereinafter "nitriles"), are well known in the container industry. For example, polyethylene terephthalate (PET) has a wide range of applications in the field of containers for foodstuffs, flavoring materials, cosmetics, beverages and so on. PET can be molded, by orientation-blowing, into transparent thin-walled containers having a high stiffness, impact strength and improved hygienic qualities with a high molding accuracy. Strong, transparent and substantially heat resistant containers may be produced by the biaxial-orientation blow-molding process in which a parison is oriented both laterally and longitudinally in a temperature range suitable for such orientation. Nitrile and heat-set PET containers are particularly heat resistant. Biaxially-oriented blow-molded containers have greater stiffness and strength as well as improved gas barrier properties and transparency.
When a thermoplastic container is filled with a hot liquid (such as a liquid sterilized at a high temperature) and sealed, subsequent thermal contraction of the liquid upon cooling results in a partial evacuation of the container which tends to deform the container walls. Backflow into a filling mechanism and the use of vacuum filling equipment during filling operations can similarly create a partial vacuum inside the container resulting in its deformation. Such deformation typically concentrates at the mechanically weaker portions of the container, resulting in an irregular and commercially unacceptable appearance. Further, if the deformation occurs in an area where the label is attached to the container, the appearance of the label may be adversely affected as a result of container deformation.
By increasing the wall thickness of the container it is possible to some extent to strengthen the container walls and thus decrease the effects of vacuum deformation. However, increasing the wall thickness results in a substantial increase in the amount of raw materials required to produce the container and a substantial decrease in production speed. The resultant costs are not acceptable to the container industry.
Prior art approaches have included the use of collapse panels to overcome thermal deformation; however, problems have developed in containers designed with large collapse panels. While large collapse panels accommodate a greater degree of controlled deformation, as the width of the collapse panel increases the strength of the container body decreases. Additionally, the wider the collapse panel the more difficult it is to mold the container. Wide collapse panels often distort when the container is removed from a heated mold in which they are formed.
As has been heretofore recognized, a thermoplastic container will have different degrees of molecular orientation along its axial dimension. During fabrication of the container a parison will be stretched in varying degrees to form the various sections of the container. For example, the portions of the parison forming the bottom and shoulder sections of the container will be stretched to a lesser extent than the portion of the parison forming the body section of the container. Also, the portion of the parison forming the body section of the container other than the collapse panel sections will undergo a slightly greater amount of stretching than the portion forming the collapse panels. That is, a greater amount of stretching will occur in the portion of the parison forming the outermost part of the body section of the container.
A prior attempt to reduce the effects of varying degrees of molecular orientation along a container's axial dimension is disclosed in U.S. Pat. No. 4,233,022 to Brady et al. Brady et al. discloses a method and apparatus for developing a strain crystallized morphology in thermoplastic containers by a heat treatment process subsequent to the blow-molding operation. This heat treatment process includes differentially heating the blow molded container along its length so that only those portions which have been significantly molecularly oriented are heat treated.
Thermoplastic containers having different degrees of molecular orientation along their axial dimensions must be fabricated in such a manner that the varying degrees of molecular orientation will not affect the physical strength, appearance or ease of manufacture of the container. The present invention is directed to a method and apparatus for effectuating the above mentioned requirements.